1. Field of the Invention
The present invention relates to a method of producing an azo pigment, more particularly to a method of producing an azo pigment which can be used to prepare printing inks of excellent quality.
2. The Prior Art
Heretofore, there have been proposed a number of methods of treating pigments with surface active agents, resins and various additives in an attempt to provide pigments for use in printing inks. Such methods include, for example, adding to pigments amphoteric surface active agents, anionic surface active agents, emulsions or the like, or treating pigments with aliphatic amines (see, for example, Japanese Patent Publication No. 1821/1973). Although the former method is effective in improving the gloss of the printing ink, the thus-produced pigment is disadvantageous in that the use thereof reduces the flowability of the resulting printing inks. On the other hand, the pigment obtained by the latter method improves the adaptability of printing inks but has drawbacks, in that, after formulation of a printing ink, a substantial degree of discoloration occurs as time goes on, leading to a reduction in brightness, gloss and color strength.
Furthermore, when pigments obtained by the previously proposed methods are used to make a gravure printing ink and the resulting ink is printed on cheap paper, ink penetration occurs ("strike-through"), ink penetrating to the opposite side of the paper sheet, thus presenting the problem that the printed matter is stained or that duplex printing is not possible. In recent years, there has been a strong demand for lowering the cost of producing prints by using cheap paper instead of expensive coated paper of high quality. Accordingly, there is a long-felt want and demand for a pigment suitable for use in making a printing ink which is free of the problem of ink penetration, which tends to occur on cheap paper.